One common form of printed circuit board employs a dielectric epoxy resin-impregnated woven glass fiber sheet, commonly known as "prepreg", to the opposite sides of which are bonded conductive copper foil. This lamination is often called a core, or board. The copper, through various photographic processes, is etched to produce conductive paths. A common technique in the manufacturing process is to assemble a stack of boards, one on top of another, in a press lay-up. The stack is called a "book". The entire book is subjected to heat and pressure in a press, and thereafter, the individual boards are separated for further processing.
One of the subsequent processing steps is drilling holes in the board for the acceptance of conductor leads of electronic components. Drill entry and drill backup material is placed adjacent the copper layers, preparatory to drilling, to assure clean holes without burrs, etc.
One of the problems encountered in the processing cycle is caused by each of the materials having a different thermal coefficient of expansion. For example, the woven glass fabric portion of the prepreg normally has a low thermal coefficient and the epoxy portion has a high coefficient. The copper foil in the board surfaces also has a high thermal coefficient. However, since the finished boards are symmetrical, the stresses are balanced. Nevertheless, during the processing, stress and adhesion tendency are encountered.
The copper surfaces of the boards are normally separated one from another by stainless steel sheets called separator or plainishing plates. They engage the copper during the laminating process to assure that the copper foil will not be indented or scratched. When epoxy adheres to a stainless steel separator plate, it becomes extremely difficult to remove and the costly separator plates must then be cleaned and polished before they can be used again. Should the stainless steel plate bond to the copper of a board, the difference in thermal coefficients will cause warpage.
To avoid any flowing epoxy from adhering to the stainless steel plates or bonding them to the copper, the stainless steel plates are in turn, separated from the copper foil by disposable release sheets.
One conventional technique for fabricating a multi-layer book of printed circuit boards is to form a press lay-up by first laying a heavy metal sheet called a caul blade or caul plate on the press platter. The caul plate supports and mounts tooling pins. Next, clip board is laid on the caul plate. The chip board helps to distribute the heat evenly and to keep any imperfection in the caul blade from being transferred to the copper surfaces of the boards.
A clean, smooth, polished stainless steel separator plate is placed on top of the chip board. Thereafter, tooling pins are inserted into the caul plate, the chip board and separator plate. The tooling pins aid in aligning the copper and prepreg components of each board. A release sheet is next laid over the stainless steel plate. The release sheets sometimes have a tendency to adhere to the copper foil, resulting in wrinkling or staining of the foil.
The next step is to begin the assembly of the boards themselves. A first layer of copper is placed over the tooling pins, on top of the release sheet. A layer of prepreg is placed over the first copper layer and a second copper layer over the prepreg. If desired, an etched and oxide treated inner layer may be laid on top of the first layer of prepreg and to this, another layer of prepreg is added. Finally, the outer layer of copper is placed on top of the prepreg. Sometimes the copper layers are pre-coated or pre-laminated, and in this case, the lamentation is placed copper side up on the prepeg. Another release sheet is laid on top of the copper, another separator plate is cleaned and positioned on top of the release sheet. This constitutes the laminations required for one board. Thus, there are two stainless steel separator plates, one on the bottom, and one on top of the assemblage, and two disposable release sheets between the steel and the copper. The procedure continues until at least four boards are assembled, one on top of another. Sometimes eight or more boards are laid up.
Upon positioning the last separator plate, another layer of chip board is added, with an upper caul plate over the entire stack. The entire assemblage is often referred to as a multi-layer book. It is then sent to the curing press.
The operating temperature and pressure of the press is set, and the press activated. The bonding-curing process takes from about one to two hours, depending upon the manufacturer's individual specifications. When bonding has taken place, and the press cooled down, the multi-layer book is removed, and the individual boards are separated. They are then covered, top and bottom with drill entry and drill back up material, and sent off for drilling.
Before the stainless steel separator plates can be used again, the epoxy must be removed, they must be sanded free of defects and scrubbed clean. The epoxy must be cleaned from the tooling pin holes. Should any scratch or deep depression be found in the separator plates, they must be sent to a machine shop for refinishing or be discarded.
It will be obvious from the foregoing that the process is highly labor intensive, the separator sheets are costly, and large quantities of release sheets are being consumed in each cycle.
It is accordingly, an object of this invention to eliminate the need for costly stainless steel separator plates between boards.
It is another object of the invention to eliminate the need for disposable release sheets.
Yet another object is to eliminate the need for separate drill entry and back up material to be applied to the top and bottom of each board preparatory to drilling.